xiv] 



ANTEDON 



363 



joint of the stem, and then five rows of 

 plates, called radials, radiating from it. 

 These five rows show us that here, as in 

 most other Echinoderms, we have to do 

 with five primitive arms or radii ; these 

 radii, however, bifurcate the moment they 

 become free from the disc, and so there 

 are ten arms : the uppermost plate in each 

 of the five rows having a double facet, on 

 to which fits the lowest of the rows of 

 plates supporting the arms. 



The arms really bifurcate again and 

 again, but in each case one of the forks 

 does not develop further and forms a 

 pinnule. If in the case of any of the 

 bifurcations both forks were to develop 

 we should have an increase in the number 

 of arms, and indeed species of Antedon 

 with twenty, forty and even one hundred 

 arms are known. 



There is no madreporite, but the whole 

 of the upper soft integument is riddled 

 with isolated pores which lead into the 

 body-cavity and are lined by ciliated cells. 

 The water- vascular ring has hanging down 

 from it a large number of stone-canals, 

 which also open freely into the body- 

 cavity. Only one pore and one stone- 

 canal exist in the stalked young, but their 

 position, comparatively near the mouth, 

 is utterly different from that in any other 

 Echinoderm. In the adult the cavity of 

 the coelom is traversed in every direction 

 by cellular cords called trabeculae. 



The anus is situated on the ventral 

 side of the body in an interradius, the 

 alimentary canal being coiled in a simple 

 spire in the disc. We have spoken above 

 of the ambulacral grooves being lined by 

 nervous cells, like those forming the radial 

 nerve in star-fish. This is indeed so, but 



FIG. 171. Rhizocrimts. 

 x about 2 . From Sars. 



